Fresh raisins and other dried fruits, even when sealed in waxed paper and foil within paperboard containers, progressively lose their softness and become hard, presumably because of moisture losses and chemical changes. The loss of softness or tenderness becomes more pronounced when the raisins are packed with dry food products, such as toasted bran or corn flakes, which have a low moisture content relative to the raisins. In order to prevent rapid loss of moisture from the raisins to the cereal, it previously has been necessary to increase the moisture content of the cereal prior to packaging with raisins. The addition of moisture to the cereal, however, causes a substantial loss of crispness, which is undesirable in many such products.
An alternative method of keeping raisins soft is to employ an edible humectant, such as glycerol. However, the simple topical application of a humectant to a dried fruit, such as raisins, having a relatively tough outer skin does not reliably increase the softness retention characteristics of the fruit under storage conditions, and the fruit will absorb only small amounts of the humectant through the skin. Moreover, any improvement in softness retention resulting from the initial incorporation of a high level of moisture into the fruit is quickly lost during conventional storage.
There are various known methods for the treatment of raisins and other dried fruits which involve infusion of edible humectants into said fruits. These methods suffer from one or more disadvantages, including the necessity of a pretreatment step using acids, alkalis, surfactants, and similar additives which necessitate one or more specific removal steps. In other methods of infusion, the total amount of humectant which can possibly be added to raisins is generally below 10% by weight. Still other methods permit the infusion of higher levels of humectant but these methods generally suffer lack of uniformity--i.e. humectant levels vary in an unsatisfactorily broad degree--or the methods call for a process which requires two or three days, or more, for completion.
U.S. Pat. No. 4,696,824 discloses a process of coating a surface of raisins with glycerol and oil in order to improve their bulk handling and bulk transportation characteristics. The surface treatment of raisins with humectant, such as glycerol, has long been known but, as noted above, surface treatment alone does not prevent the undesirable drying out of raisins when packaged with dry products such as ready-to-eat cereals.
U.S. Pat. Nos. 3,952,112 and 4,103,035 teach methods of treating raisins with glycerol (and other polyalcohol humectants) by a process involving immersion and soaking of the raisins in glycerol or a glycerol-containing solution for a period of about 24 hours. Pretreatment of the raisins is ordinarily required. U.S. Pat. No. 3,952,112 teaches various pretreatment steps which enhance the ability of the raisins to absorb the glycerol during the soaking process. Such pretreatment steps include soaking the raisins in water, application of a vacuum, treating with small amounts of a surface active agent or a weak alkali, etc. In U.S. Pat. No. 4,103,035 the suggested pretreatment steps include soaking in water and treatment with an acidulating agent. By means of the processes taught in these patents, and with proper choice of reaction times and conditions, it has been possible to attain high levels of glycerol in raisins. However, under industrial conditions, the glycerol content of the raisins has varied considerably. The result is that not all of the raisins attained the desired high levels of glycerol infusion and therefore, when packaged with dry cereals, some of these dried out to an unsatisfactory degree.
U.S. Pat. Nos. 4,917,910 and 5,000,971 disclose a method of infusing high levels of glycerol into raisins by means of an initial step involving the spraying of glycerol onto a tumbling bed of raisins. It appears, however, that in order to attain a high level of glycerol infusion the raisins have to then be maintained in a static condition for a period of time of about four weeks.
Co-assigned U.S. patent application No. 07/917,834 discloses a process for infusing humectant into raisins under conditions which reduce the period of processing time to about 24 hours. The process involves a tempering step in which the raisins are maintained in a specified temperature range. However, it would still be desirable to develop a process in which humectant infusion can be done in a shorter period.
It is the principal object of this invention to provide an improved process for infusing high levels of glycerol into raisins by a method which does not require substantially more than about six hours of processing time and results in a product having a substantially uniform level of glycerol infusion.